This proposal is designed to increase our understanding of the genetic consequences of x-rays by the determination of the precise nature of the mutations at the DNA sequence level. This includes the analysis of point mutations, deletions and rearrangements. The experiment are designed to permit the reconstruction of chromosomal break points within and exterior to the target gene. The mutational target is the CHO aprt gene which will be studied both as an endogenous gene, as well as in a retroviral based shuttle vector. This comparison will provide a validation for the more rapid shuttle vector methodology. The effect of a high dose (greater than 10 fold mutation induction over background) and a low dose (approximately the doubling dose), as well as dose fractionation will be studied.